Network Working Group N. Cam-Winget
Request for Comments: 5422 D. McGrew
Category: Informational J. Salowey
H. Zhou
Cisco Systems
March 2009
Dynamic Provisioning Using Flexible Authentication viaSecure Tunneling Extensible Authentication Protocol (EAP-FAST)
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Please review these documents carefully, as they describe your rights
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Contributions published or made publicly available before November
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material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
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not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
IESG Note
EAP-FAST has been implemented by many vendors and it is used in the
Internet. Publication of this specification is intended to promote
interoperability by documenting current use of existing EAP methods
within EAP-FAST.
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RFC 5422 Dynamic Provisioning Using EAP-FAST March 2009
The EAP method EAP-FAST-MSCHAPv2 reuses the EAP type code assigned to
EAP-MSCHAPv2 (26) for authentication within an anonymous TLS tunnel.
In order to minimize the risk associated with an anonymous tunnel,
changes to the method were made that are not interoperable with EAP-
MSCHAPv2. Since EAP-MSCHAPv2 does not support method-specific
version negotiation, the use of EAP-FAST-MSCHAPv2 is implied by the
use of an anonymous EAP-FAST tunnel. This behavior may cause
problems in implementations where the use of unaltered EAP-MSCHAPv2
is needed inside an anonymous EAP-FAST tunnel. Since such support
requires special case execution of a method within a tunnel, it also
complicates implementations that use the same method code both within
and outside of the tunnel method. If EAP-FAST were to be designed
today, these difficulties could be avoided by utilization of unique
EAP Type codes. Given these issues, assigned method types must not
be re-used with different meaning inside tunneled methods in the
future.
Abstract
The Flexible Authentication via Secure Tunneling Extensible
Authentication Protocol (EAP-FAST) method enables secure
communication between a peer and a server by using Transport Layer
Security (TLS) to establish a mutually authenticated tunnel. EAP-
FAST also enables the provisioning credentials or other information
through this protected tunnel. This document describes the use of
EAP-FAST for dynamic provisioning.
Table of Contents
1. Introduction ....................................................41.1. Specification Requirements .................................41.2. Terminology ................................................42. EAP-FAST Provisioning Modes .....................................53. Dynamic Provisioning Using EAP-FAST Conversation ................63.1. Phase 1 TLS Tunnel .........................................73.1.1. Server-Authenticated Tunnel .........................73.1.2. Server-Unauthenticated Tunnel .......................73.2. Phase 2 - Tunneled Authentication and Provisioning .........73.2.1. Server-Authenticated Tunneled Authentication ........83.2.2. Server-Unauthenticated Tunneled Authentication ......83.2.3. Authenticating Using EAP-FAST-MSCHAPv2 ..............8
3.2.4. Use of Other Inner EAP Methods for EAP-FAST
Provisioning ........................................9
3.3. Key Derivations Used in the EAP-FAST Provisioning
Exchange ..................................................103.4. Peer-Id, Server-Id, and Session-Id ........................113.5. Network Access after EAP-FAST Provisioning ................114. Information Provisioned in EAP-FAST ............................12Cam-Winget, et al. Informational [Page 2]

RFC 5422 Dynamic Provisioning Using EAP-FAST March 20091. Introduction
EAP-FAST [RFC4851] is an EAP method that can be used to mutually
authenticate the peer and server. Credentials such as a pre-shared
key, certificate trust anchor, or a Protected Access Credential (PAC)
must be provisioned to the peer before it can establish mutual
authentication with the server. In many cases, the provisioning of
such information presents deployment hurdles. Through the use of the
protected TLS [RFC5246] tunnel, EAP-FAST can enable dynamic in-band
provisioning to address such deployment obstacles.
1.1. Specification Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
1.2. Terminology
Much of the terminology used in this document comes from [RFC3748].
The terms "peer" and "server" are used interchangeably with the terms
"EAP peer" and "EAP server", respectively. Additional terms are
defined below:
Man in the Middle (MITM)
An adversary that can successfully inject itself between a peer
and EAP server. The MITM succeeds by impersonating a valid peer
or server.
Provisioning
Providing a peer with a trust anchor, shared secret, or other
appropriate information needed to establish a security
association.
Protected Access Credential (PAC)
Credentials distributed to a peer for future optimized network
authentication. The PAC consists of at most three components: a
shared secret, an opaque element, and optional information. The
shared secret part contains the secret key shared between the peer
and server. The opaque part contains the shared secret encrypted
by a private key only known to the server. It is provided to the
peer and is presented back to the server when the peer wishes to
obtain access to network resources. Finally, a PAC may optionally
include other information that may be useful to the peer.
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RFC 5422 Dynamic Provisioning Using EAP-FAST March 2009
Tunnel PAC
A set of credentials stored by the peer and consumed by both the
peer and the server to establish a TLS tunnel.
User Authorization PAC
A User Authorization PAC is server-encrypted data containing
authorization information associated with a previously
authenticated user. The User Authorization PAC does not contain a
key, but rather it is generally bound to a Tunnel PAC, which is
used with the User Authorization PAC.
Machine Authentication PAC
A Machine Authentication PAC contains server-encrypted data
containing authorization information associated with a device. A
Machine Authentication PAC may be used instead of a Tunnel PAC to
establish the TLS tunnel to provide machine authentication and
authorization information. The Machine Authentication PAC is
useful in cases where the machine needs to be authenticated and
authorized to access a network before a user has logged in.
2. EAP-FAST Provisioning Modes
EAP-FAST supports two modes for provisioning:
1. Server-Authenticated Provisioning Mode - Provisioning inside a
TLS tunnel that provides server-side authentication.
2. Server-Unauthenticated Provisioning Mode - Provisioning inside an
anonymous TLS tunnel.
The EAP-FAST provisioning modes use EAP-FAST phase 2 inside a secure
TLS tunnel established during phase 1. [RFC4851] describes the EAP-
FAST phases in greater detail.
In the Server-Authenticated Provisioning Mode, the peer has
successfully authenticated the EAP server as part of EAP-FAST phase 1
(i.e., TLS tunnel establishment). Additional exchanges MAY occur
inside the tunnel to allow the EAP server to authenticate the EAP
peer before provisioning any information.
In the Server-Unauthenticated Provisioning Mode, an unauthenticated
TLS tunnel is established in the EAP-FAST phase 1. The peer MUST
negotiate a TLS anonymous Diffie-Hellman-based ciphersuite to signal
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RFC 5422 Dynamic Provisioning Using EAP-FAST March 2009
that it wishes to use Server-Unauthenticateded Provisioning Mode.
This provisioning mode enables the bootstrapping of peers where the
peer lacks strong credentials usable for mutual authentication with
the server.
Since the server is not authenticated in the Server-Unauthenticated
Provisioning Mode, it is possible that an attacker may intercept the
TLS tunnel. If an anonymous tunnel is used, then the peer and server
MUST negotiate and successfully complete an EAP method supporting
mutual authentication and key derivation as described in Section 6.
The peer then uses the Crypto-Binding TLV to validate the integrity
of the TLS tunnel, thereby verifying that the exchange was not
subject to a man-in-the-middle attack.
Server-Authenticated Provisioning Mode protects against the man-in-
the-middle attack; however, it requires provisioning the peer with
the credentials necessary to authenticate the server. Environments
willing to trade off the security risk of a man-in-the-middle attack
for ease of deployment can choose to use the Server-Unauthenticated
Provisioning Mode.
Assuming that an inner EAP method and Crypto-Binding TLV exchange is
successful, the server will subsequently provide credential
information, such as a shared key using a PAC TLV or the trusted
certificate root(s) of the server using a Server-Trusted-Root TLV.
Once the EAP-FAST Provisioning conversation completes, the peer is
expected to use the provisioned credentials in subsequent EAP-FAST
authentications.
3. Dynamic Provisioning Using EAP-FAST Conversation
The provisioning occurs in the following steps, which are detailed in
the subsequent sections and in RFC 4851. First, the EAP-FAST phase 1
TLS tunnel is established. During this process, extra material is
extracted from the TLS key derivation for use as challenges in the
subsequent authentication exchange. Next, an inner EAP method, such
as EAP-FAST-MSCHAPv2 (Microsoft Challenge Handshake Authentication
Protocol version 2), is executed within the EAP-FAST phase 2 TLS
tunnel to authenticate the client using the challenges derived from
the phase 1 TLS exchange. Following successful authentication and
Crypto-Binding TLV exchange, the server provisions the peer with PAC
information including the secret PAC-Key and the PAC-Opaque.
Finally, the EAP-FAST conversation completes with Result TLV
exchanges defined in RFC 4851. The exported EAP Master Session Key
(MSK) and Extended MSK (EMSK) are derived from a combination of the
tunnel key material and key material from the inner EAP method
exchange.
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RFC 5422 Dynamic Provisioning Using EAP-FAST March 20093.1. Phase 1 TLS Tunnel3.1.1. Server-Authenticated Tunnel
The provisioning EAP-FAST exchange uses the same sequence as the EAP-
FAST authentication phase 1 to establish a protected TLS tunnel.
Implementations supporting this version of the Sever-Authenticated
Provisioning Mode MUST support the following TLS ciphersuites defined
in [RFC5246]:
TLS_RSA_WITH_RC4_128_SHA
TLS_RSA_WITH_AES_128_CBC_SHA
TLS_DHE_RSA_WITH_AES_128_CBC_SHA
Other TLS ciphersuites that provide server authentication and
encryption MAY be supported. The server MAY authenticate the peer
during the TLS handshake in Server-Authenticated Provisioning Mode.
To adhere to best security practices, the peer MUST validate the
server's certificate chain when performing server-side authentication
to obtain the full security benefits of Server-Authenticated
provisioning.
3.1.2. Server-Unauthenticated Tunnel
Implementations supporting this version of the Sever-Unauthenticated
Provisioning Mode MUST support the following TLS ciphersuite defined
in [RFC5246]:
TLS_DH_anon_WITH_AES_128_CBC_SHA
Anonymous ciphersuites SHOULD NOT be allowed outside of EAP-FAST
Server-Unauthenticated Provisioning Mode. Any ciphersuites that are
used for Server-Unauthenticated Provisioning Mode MUST provide a key
agreement contributed by both parties. Therefore, ciphersuites based
on RSA key transport MUST NOT be used for this mode. Ciphersuites
that are used for provisioning MUST provide encryption.
3.2. Phase 2 - Tunneled Authentication and Provisioning
Once a protected tunnel is established and the server is
unauthenticated, the peer and server MUST execute additional
authentication and perform integrity checks of the TLS tunnel. Even
if both parties are authenticated during TLS tunnel establishment,
the peer and server MAY wish to perform additional authentication
within the tunnel. As defined in [RFC4851], the authentication
exchange will be followed by an Intermediate-Result TLV and a Crypto-
Binding TLV, if the EAP method succeeded. The Crypto-Binding TLV
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RFC 5422 Dynamic Provisioning Using EAP-FAST March 2009
provides a check on the integrity of the tunnel with respect to the
endpoints of the EAP method. If the preceding is successful, then a
provisioning exchange MAY take place. The provisioning exchange will
use a PAC TLV exchange if a PAC is being provisioned and a Server-
Trusted-Root TLV if a trusted root certificate is being provisioned.
The provisioning MAY be solicited by the peer or it MAY be
unsolicited. The PAC TLV exchange consists of the server
distributing the PAC in a corresponding PAC TLV to the peer and the
peer confirming its receipt in a final PAC TLV Acknowledgement
message. The peer may also use the PAC TLV to request that the
server send a PAC. The provisioning TLVs MAY be piggybacked onto the
Result TLV. Many implementations process TLVs in the order they are
received; thus, for proper provisioning to occur, the Result TLV MUST
precede the TLVs to be provisioned (e.g., Tunnel PAC, Machine
Authentication PAC, and User Authorization PAC). A PAC TLV MUST NOT
be accepted if it is not encapsulated in an encrypted TLS tunnel.
A fresh PAC MAY be distributed if the server detects that the PAC is
expiring soon. In-band PAC refreshing is through the PAC TLV
mechanism. The decision of whether or not to refresh the PAC is
determined by the server. Based on the PAC-Opaque information, the
server MAY determine not to refresh a peer's PAC, even if the PAC-Key
has expired.
3.2.1. Server-Authenticated Tunneled Authentication
If Server-Authenticated Provisioning Mode is in use, then any EAP
method may be used within the TLS tunnel to authenticate the peer
that is allowed by the peer's policy.
3.2.2. Server-Unauthenticated Tunneled Authentication
If Server-Unauthenticated Provisioning Mode is in use, then peer
authenticates the server and the server authenticates the peer within
the tunnel. The only method for performing authentication defined in
this version of EAP-FAST is EAP-FAST-MSCHAPv2 (in a special way as
described in the following section). It is possible for other
methods to be defined to perform this authentication in the future.
3.2.3. Authenticating Using EAP-FAST-MSCHAPv2
EAP-FAST-MSCHAPv2 is a specific instantiation of EAP-MSCHAPv2
[EAP-MSCHAPv2] defined for use within EAP-FAST. The 256-bit inner
session key (ISK) is generated from EAP-FAST-MSCHAPv2 by combining
the 128-bit master keys derived according to RFC 3079 [RFC3079], with
the MasterSendKey taking the first 16 octets and MasterReceiveKey
taking the last 16 octets.
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RFC 5422 Dynamic Provisioning Using EAP-FAST March 2009
Implementations of this version of the EAP-FAST Server-
Unauthenticated Provisioning Mode MUST support EAP-FAST-MSCHAPv2 as
the inner authentication method. While other authentication methods
exist, EAP-FAST-MSCHAPv2 was chosen for several reasons:
o It provides the ability to slow an active attack by using a hash-
based challenge-response protocol.
o Its use of a challenge-response protocol, such as MSCHAPv2,
provides some ability to detect a man-in-the-middle attack during
Server-Unauthenticated Provisioning Mode.
o It is already supported by a large deployed base.
o It allows support for password change during the EAP-FAST
provisioning modes.
When using an anonymous Diffie-Hellman (DH) key agreement, the
challenges MUST be generated as defined in Section 3.3. This forms a
binding between the tunnel and the EAP-FAST-MSCHAPv2 exchanges by
using keying material generated during the EAP-FAST tunnel
establishment as the EAP-FAST-MSCHAPv2 challenges instead of using
the challenges exchanged within the protocol itself. The exchanged
challenges are zeroed upon transmission, ignored upon reception, and
the challenges derived from the TLS key exchange are used in the
calculations. When EAP-FAST-MSCHAPv2 is used within a tunnel
established using a ciphersuite other than one that provides
anonymous key agreement, the randomly generated EAP-FAST-MSCHAPv2
challenges MUST be exchanged and used.
The EAP-FAST-MSCHAPv2 exchange forces the server to provide a valid
ServerChallengeResponse, which must be a function of the server
challenge, peer challenge, and password as part of its response.
This reduces the window of vulnerability of a man-in-the-middle
attack spoofing the server by requiring the attacker to successfully
break the password within the peer's challenge-response time limit.
3.2.4. Use of Other Inner EAP Methods for EAP-FAST Provisioning
Once a protected tunnel is established, typically the peer
authenticates itself to the server before the server can provision
the peer. If the authentication mechanism does not support mutual
authentication and protection from man-in-the-middle attacks, then
Server-Authenticated Provisioning Mode MUST be used. Within a server
side, authenticated tunnel authentication mechanisms such as EAP-
FAST-GTC (Generic Token Card) [RFC5421] MAY be used. This will
enable peers using other authentication mechanisms such as password
database and one-time passwords to be provisioned in-band as well.
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RFC 5422 Dynamic Provisioning Using EAP-FAST March 2009
This version of the EAP-FAST provisioning mode implementation MUST
support both EAP-FAST-GTC and EAP-FAST-MSCHAPv2 within the tunnel in
Server-Authenticated Provisioning Mode.
It should be noted that Server-Authenticated Provisioning Mode
provides significant security advantages over Server-Unauthenticated
Provisioning Mode even when EAP-FAST-MSCHAPv2 is being used as the
inner method. It protects the EAP-FAST-MSCHAPv2 exchanges from
potential active MITM attacks by verifying the server's authenticity
before executing EAP-FAST-MSCHAPv2. Server-Authenticated
Provisioning Mode is the recommended provisioning mode. The EAP-FAST
peer MUST use the Server- Authenticated Provisioning Mode whenever it
is configured with a valid trust root for a particular server.
3.3. Key Derivations Used in the EAP-FAST Provisioning Exchange
The TLS tunnel key is calculated according to the TLS version with an
extra 72 octets of key material derived from the end of the
key_block. Portions of the extra 72 octets are used for the EAP-FAST
provisioning exchange session key seed and as the random challenges
in the EAP-FAST-MSCHAPv2 exchange.
To generate the key material, compute:
key_block = PRF(master_secret,
"key expansion",
server_random +
client_random);
until enough output has been generated.
For example, the key_block for TLS 1.0 [RFC2246] is partitioned as
follows:
client_write_MAC_secret[hash_size]
server_write_MAC_secret[hash_size]
client_write_key[Key_material_length]
server_write_key[key_material_length]
client_write_IV[IV_size]
server_write_IV[IV_size]
session_key_seed[40]
ServerChallenge[16]
ClientChallenge[16]
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RFC 5422 Dynamic Provisioning Using EAP-FAST March 2009
and the key_block for subsequent versions is partitioned as follows:
client_write_MAC_secret[hash_size]
server_write_MAC_secret[hash_size]
client_write_key[Key_material_length]
server_write_key[key_material_length]
session_key_seed[40]
ServerChallenge[16]
ClientChallenge[16]
In the extra key material, session_key_seed is used for the EAP-FAST
Crypto-Binding TLV exchange while the ServerChallenge and
ClientChallenge correspond to the authentication server's EAP-FAST-
MSCHAPv2 challenge and the peer's EAP-FAST-MSCHAPv2 challenge,
respectively. The ServerChallenge and ClientChallenge are only used
for the EAP-FAST-MSCHAPv2 exchange when Diffie-Hellman anonymous key
agreement is used in the EAP-FAST tunnel establishment.
3.4. Peer-Id, Server-Id, and Session-Id
The provisioning modes of EAP-FAST do not change the general EAP-
FAST protocol and thus how the Peer-Id, Server-Id, and Session-Id are
determined is based on the [RFC4851] techniques.
Section 3.4 of [RFC4851] describes how the Peer-Id and Server-Id are
determined; Section 3.5 describes how the Session-Id is generated.
3.5. Network Access after EAP-FAST Provisioning
After successful provisioning, network access MAY be granted or
denied depending upon the server policy. For example, in the Server-
Authenticated Provisioning Mode, access can be granted after the EAP
server has authenticated the peer and provisioned it with a Tunnel
PAC (i.e., a PAC used to mutually authenticate and establish the EAP-
FAST tunnel). Additionally, peer policy MAY instruct the peer to
disconnect the current provisioning connection and initiate a new
EAP-FAST exchange for authentication utilizing the newly provisioned
information. At the end of the Server-Unauthenticated Provisioning
Mode, network access SHOULD NOT be granted as this conversation is
intended for provisioning only and thus no network access is
authorized. The server MAY grant access at the end of a successful
Server-Authenticated provisioning exchange.
If after successful provisioning access to the network is denied, the
EAP Server SHOULD conclude with an EAP Failure. The EAP server SHALL
NOT grant network access or distribute any session keys to the
Network Access Server (NAS) if this exchange is not intended to
provide network access. Even though the provisioning mode completes
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RFC 5422 Dynamic Provisioning Using EAP-FAST March 2009
with a successful inner termination (e.g., a successful Result TLV),
the server policy defines whether or not the peer gains network
access. Thus, it is feasible that the server, while providing a
successful Result TLV, may conclude that its authentication policy
was not satisfied and terminate the conversation with an EAP Failure.
Denying network access after EAP-FAST Provisioning may cause
disruption in scenarios such as wireless devices (e.g., in IEEE
802.11 devices, an EAP Failure may trigger a full 802.11
disassociation). While a full EAP restart can be performed, a smooth
transition to the subsequent EAP-FAST authentications to enable
network access can be achieved by the peer or server initiating TLS
renegotiation, where the newly provisioned credentials can be used to
establish a server-authenticated or mutually authenticated TLS tunnel
for authentication. Either the peer or server may reject the request
for TLS renegotiation. Upon completion of the TLS negotiation and
subsequent authentication, normal network access policy on EAP-FAST
authentication can be applied.
4. Information Provisioned in EAP-FAST
Multiple types of credentials MAY be provisioned within EAP-FAST.
The most common credential is the Tunnel PAC that is used to
establish the EAP-FAST phase 1 tunnel. In addition to the Tunnel
PAC, other types of credentials and information can also be
provisioned through EAP-FAST. They may include trusted root
certificates, PACs for specific purposes, and user identities, to
name a few. Typically, provisioning is invoked after both the peer
and server authenticate each other and after a successful Crypto-
Binding TLV exchange. However, depending on the information being
provisioned, mutual authentication MAY not be needed.
At a minimum, either the peer or server must prove authenticity
before credentials are provisioned to ensure that information is not
freely provisioned to or by adversaries. For example, the EAP server
may not need to authenticate the peer to provision it with trusted
root certificates. However, the peer SHOULD authenticate the server
before it can accept a trusted server root certificate.
4.1. Protected Access Credential
A Protected Access Credential (PAC) is a security credential
generated by the server that holds information specific to a peer.
The server distributes all PAC information through the use of a PAC
TLV. Different types of PAC information are identified through the
PAC Type and other PAC attributes defined in this section. This
document defines three types of PACs: a Tunnel PAC, a Machine
Authentication PAC, and a User Authorization PAC.
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RFC 5422 Dynamic Provisioning Using EAP-FAST March 20094.1.1. Tunnel PAC
The server distributes the Tunnel PAC to the peer, which uses it in
subsequent attempts to establish a secure EAP-FAST TLS tunnel with
the server. The Tunnel PAC includes a secret key (PAC-Key), data
that is opaque to the peer (PAC-Opaque), and other information (PAC-
Info) that the peer can interpret. The opaque data is generated by
the server and cryptographically protected so it cannot be modified
or interpreted by the peer. The Tunnel PAC conveys the server policy
of what must and can occur in the protected phase 2 tunnel. It is up
to the server policy to include what is necessary in a PAC-Opaque to
enforce the policy in subsequent TLS handshakes. For example, user
identity, I-ID, can be included as the part of the server policy.
This I-ID information limits the inner EAP methods to be carried only
on the specified user identity. Other types of information can also
be included, such as which EAP method(s) and which TLS ciphersuites
are allowed. If the server policy is not included in a PAC-Opaque,
then there is no limitation imposed by the PAC on the usage of the
inner EAP methods or user identities inside the tunnel established by
the use of that PAC.
4.1.2. Machine Authentication PAC
The Machine Authentication PAC contains information in the PAC-Opaque
that identifies the machine. It is meant to be used by a machine
when network access is required and no user is logged in. Typically,
a server will only grant the minimal amount of access required for a
machine without a user present based on the Machine Authentication
PAC. The Machine Authentication PAC MAY be provisioned during the
authentication of a user. It SHOULD be stored by the peer in a
location that is only accessible to the machine. This type of PAC
typically persists across sessions.
The peer can use the Machine Authentication PAC as the Tunnel PAC to
establish the TLS tunnel. The EAP server MAY have a policy to bypass
additional inner EAP method and grant limited network access based on
information in the Machine Authentication PAC. The server MAY
request additional exchanges to validate machine's other
authorization criteria, such as posture information etc., before
granting network access.
4.1.3. User Authorization PAC
The User Authorization PAC contains information in the PAC-Opaque
that identifies a user and provides authorization information. This
type of PAC does not contain a PAC-Key. The PAC-Opaque portion of
the User Authorization PAC is presented within the protected EAP-FAST
TLS tunnel to provide user information during stateless session
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RFC 5422 Dynamic Provisioning Using EAP-FAST March 2009
resume so user authentication MAY be skipped. The User Authorization
PAC MAY be provisioned after user authentication. It is meant to be
short lived and not persisted across logon sessions. The User
Authorization PAC SHOULD only be available to the user for which it
is provisioned. The User Authorization PAC SHOULD be deleted from
the peer when the local authorization state of a user's session
changes, such as upon the user logs out.
Once the EAP-FAST phase 1 TLS tunnel is established, the peer MAY
present a User Authorization PAC to the server in a PAC TLV. This is
sent as TLS application data, but it MAY be included in the same
message as the Finished Handshake message sent by the peer. The User
Authorization PAC MUST only be sent within the protection of an
encrypted tunnel to an authenticated entity. The server will decrypt
the PAC and evaluate the contents. If the contents are valid and the
server policy allows the session to be resumed based on this
information, then the server will complete the session resumption and
grant access to the peer without requiring an inner authentication
method. This is called stateless session resume in EAP-FAST. In
this case, the server sends the Result TLV indicating success without
the Crypto-Binding TLV and the peer sends back a Result TLV
indicating success. If the User Authorization PAC fails the server
validation or the server policy, the server MAY either reject the
request or continue with performing full user authentication within
the tunnel.
4.1.4. PAC Provisioning
To request provisioning of a PAC, a peer sends a PAC TLV containing a
PAC attribute of PAC Type set to the appropriate value. For a Tunnel
PAC, the value is '1'; for a Machine Authentication PAC, the value is
'2'; and for a User Authorization PAC, the value is '3'. The request
MAY be issued after the peer has determined that it has successfully
authenticated the EAP server and validated the Crypto-Binding TLV to
ensure that the TLS tunnel's integrity is intact. Since anonymous DH
ciphersuites are only allowed for provisioning a Tunnel PAC, if an
anonymous ciphersuite is negotiated, the Tunnel PAC MAY be
provisioned automatically by the server. The peer MUST send separate
PAC TLVs for each type of PAC it wants to provision. Multiple PAC
TLVs can be sent in the same packet or different packets. When
requesting the Machine Authentication PAC, the peer SHOULD include an
I-ID TLV containing the machine name prefixed by "host/". The EAP
server will send the PACs after its internal policy has been
satisfied, or it MAY ignore the request or request additional
authentications if its policy dictates. If a peer receives a PAC
with an unknown type, it MUST ignore it.
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RFC 5422 Dynamic Provisioning Using EAP-FAST March 2009
A PAC-TLV containing PAC-Acknowledge attribute MUST be sent by the
peer to acknowledge the receipt of the Tunnel PAC. A PAC-Acknowledge
TLV MUST NOT be used by the peer to acknowledge the receipt of other
types of PACs.
Please see Appendix A.1 for an example of packet exchanges to
provision a Tunnel PAC.
4.2. PAC TLV Format
The PAC TLV provides support for provisioning the Protected Access
Credential (PAC) defined within [RFC4851]. The PAC TLV carries the
PAC and related information within PAC attribute fields.
Additionally, the PAC TLV MAY be used by the peer to request
provisioning of a PAC of the type specified in the PAC Type PAC
attribute. The PAC TLV MUST only be used in a protected tunnel
providing encryption and integrity protection. A general PAC TLV
format is defined as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PAC Attributes...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
0 - Non-mandatory TLV
1 - Mandatory TLV
R
Reserved, set to zero (0)
TLV Type
11 - PAC TLV
Length
Two octets containing the length of the PAC attributes
field in octets.
PAC Attributes
A list of PAC attributes in the TLV format.
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RFC 5422 Dynamic Provisioning Using EAP-FAST March 2009
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Key ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
1 - PAC-Key
Length
2-octet length indicating a 32-octet key
Key
The value of the PAC-Key.
4.2.3. PAC-Opaque
The PAC-Opaque attribute is included within the PAC TLV whenever the
server wishes to issue or renew a PAC or the client wishes to present
a User Authorization PAC to the server.
The PAC-Opaque is opaque to the peer and thus the peer MUST NOT
attempt to interpret it. A peer that has been issued a PAC-Opaque by
a server stores that data and presents it back to the server
according to its PAC Type. The Tunnel PAC is used in the ClientHello
SessionTicket extension field defined in [RFC5077]. If a peer has
opaque data issued to it by multiple servers, then it stores the data
issued by each server separately according to the A-ID. This
requirement allows the peer to maintain and use each opaque datum as
an independent PAC pairing, with a PAC-Key mapping to a PAC-Opaque
identified by the A-ID. As there is a one-to-one correspondence
between the PAC-Key and PAC-Opaque, the peer determines the PAC-Key
and corresponding PAC-Opaque based on the A-ID provided in the EAP-
FAST/Start message and the A-ID provided in the PAC-Info when it was
provisioned with a PAC-Opaque.
The PAC-Opaque attribute format is summarized as follows:
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RFC 5422 Dynamic Provisioning Using EAP-FAST March 2009
Length
2-octet Length field containing the length of the attributes
field in octets.
Attributes
The attributes field contains a list of PAC attributes. Each
mandatory and optional field type is defined as follows:
3 - PAC-LIFETIME
This is a 4-octet quantity representing the expiration time
of the credential expressed as the number of seconds,
excluding leap seconds, after midnight UTC, January 1, 1970.
This attribute MAY be provided to the peer as part of the
PAC-Info.
4 - A-ID
The A-ID is the identity of the authority that issued the
PAC. The A-ID is intended to be unique across all issuing
servers to avoid namespace collisions. The A-ID is used by
the peer to determine which PAC to employ. The A-ID is
treated as an opaque octet string. This attribute MUST be
included in the PAC-Info attribute. The A-ID MUST match the
A-ID the server used to establish the tunnel. Since many
existing implementations expect the A-ID to be 16 octets in
length, it is RECOMMENDED that the length of an A-ID be 16
octets for maximum interoperability. One method for
generating the A-ID is to use a high-quality random number
generator to generate a 16-octet random number. An
alternate method would be to take the hash of the public key
or public key certificate belonging a server represented by
the A-ID.
5 - I-ID
Initiator identifier (I-ID) is the peer identity associated
with the credential. This identity is derived from the
inner EAP exchange or from the client-side authentication
during tunnel establishment if inner EAP method
authentication is not used. The server employs the I-ID in
the EAP-FAST phase 2 conversation to validate that the same
peer identity used to execute EAP-FAST phase 1 is also used
in at minimum one inner EAP method in EAP-FAST phase 2. If
the server is enforcing the I-ID validation on the inner EAP
method, then the I-ID MUST be included in the PAC-Info, to
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RFC 5422 Dynamic Provisioning Using EAP-FAST March 2009
enable the peer to also enforce a unique PAC for each unique
user. If the I-ID is missing from the PAC-Info, it is
assumed that the Tunnel PAC can be used for multiple users
and the peer will not enforce the unique-Tunnel-PAC-per-user
policy.
7 - A-ID-Info
Authority Identifier Information is intended to provide a
user-friendly name for the A-ID. It may contain the
enterprise name and server name in a human-readable format.
This TLV serves as an aid to the peer to better inform the
end-user about the A-ID. The name is encoded in UTF-8
[RFC3629] format. This attribute MUST be included in the
PAC-Info.
10 - PAC-type
The PAC-Type is intended to provide the type of PAC. This
attribute SHOULD be included in the PAC-Info. If the PAC-
Type is not present, then it defaults to a Tunnel PAC (Type
1).
4.2.5. PAC-Acknowledgement TLV
The PAC-Acknowledgement is used to acknowledge the receipt of the
Tunnel PAC by the peer. The peer includes the PAC-Acknowledgement
TLV in a PAC-TLV sent to the server to indicate the result of the
processing and storing of a newly provisioned Tunnel PAC. This TLV
is only used when Tunnel PAC is provisioned.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Result |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
8 - PAC-Acknowledgement
Length
The length of this field is two octets containing a value of 2.
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RFC 5422 Dynamic Provisioning Using EAP-FAST March 2009
cannot be responded to with a Negative Acknowledgement (NAK) TLV.
The Server-Trusted-Root TLV MUST only be sent as an inner TLV (inside
the protection of the tunnel).
After the peer has determined that it has successfully authenticated
the EAP server and validated the Crypto-Binding TLV, it MAY send one
or more Server-Trusted-Root TLVs (marked as optional) to request the
trusted server root certificates from the EAP server. The EAP server
MAY send one or more root certificates with a Public Key
Cryptographic System #7 (PKCS#7) TLV inside Server-Trusted-Root TLV.
The EAP server MAY also choose not to honor the request. Please see
Appendix A.3 for an example of a server provisioning a server trusted
root certificate.
4.3.1. Server-Trusted-Root TLV
The Server-Trusted-Root TLV allows the peer to send a request to the
EAP server for a list of trusted roots. The server may respond with
one or more root certificates in PKCS#7 [RFC2315] format.
If the EAP server sets the credential format to PKCS#7-Server-
Certificate-Root, then the Server-Trusted-Root TLV should contain the
root of the certificate chain of the certificate issued to the EAP
server packaged in a PKCS#7 TLV. If the Server certificate is a
self-signed certificate, then the root is the self-signed
certificate.
If the Server-Trusted-Root TLV credential format contains a value
unknown to the peer, then the EAP peer should ignore the TLV.
The Server-Trusted-Root TLV is defined as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Credential-Format | Cred TLVs...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
M
0 - Non-mandatory TLV
R
Reserved, set to zero (0)
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RFC 5422 Dynamic Provisioning Using EAP-FAST March 2009
TLV Type
18 - Server-Trusted-Root TLV [RFC4851]
Length
>=2 octets
Credential-Format
The Credential-Format field is two octets.
Values include:
1 - PKCS#7-Server-Certificate-Root
Cred TLVs
This field is of indefinite length. It contains TLVs
associated with the credential format. The peer may
leave this field empty when using this TLV to request
server trust roots.
4.3.2. PKCS#7 TLV
The PKCS#7 TLV is sent by the EAP server to the peer inside the
Server-Trusted-Root TLV. It contains PKCS#7-wrapped [RFC2315] X.509
certificates. The format consists of a certificate or certificate
chain in a Certificates-Only PKCS#7 SignedData message as defined in
[RFC2311].
The PKCS#7 TLV is always marked as optional, which cannot be
responded to with a NAK TLV. EAP-FAST server implementations that
claim to support the dynamic provisioning defined in this document
SHOULD support this TLV. EAP-FAST peer implementations MAY support
this TLV.
If the PKCS#7 TLV contains a certificate or certificate chain that is
not acceptable to the peer, then the peer MUST ignore the TLV.
The PKCS#7 TLV is defined as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PKCS #7 Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
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RFC 5422 Dynamic Provisioning Using EAP-FAST March 2009
A registry of values, named "EAP-FAST PAC Types", has been created
for PAC-Type values used in the PAC-Type TLV. The initial values
that populate the registry are:
1 - Tunnel PAC
2 - Machine Authentication PAC
3 - User Authorization PAC
Values from 4 to 63 are allocated for management by Cisco. Values 64
to 255 are assigned with a "Specification Required" policy.
A registry of values, named "EAP-FAST Server-Trusted-Root Credential
Format Types", has been created for Credential-Format values used in
the Server-Trusted-Root TLV. The initial values that populate the
registry are:
1 - PKCS#7-Server-Certificate-Root
Values from 2 to 63 are allocated for management by Cisco. Values 64
to 255 are assigned with a "Specification Required" policy.
6. Security Considerations
The Dynamic Provisioning EAP-FAST protocol shares the same security
considerations outlined in [RFC4851]. Additionally, it also has its
unique security considerations described below:
6.1. Provisioning Modes and Man-in-the-Middle Attacks
EAP-FAST can be invoked in two different provisioning modes: Server-
Authenticated Provisioning Mode and Server-Unauthenticated
Provisioning Mode. Each mode provides different levels of resistance
to man-in-the-middle attacks. The following list identifies some of
the problems associated with a man-in-the-middle attack:
o Disclosure of secret information such as keys, identities, and
credentials to an attacker
o Spoofing of a valid server to a peer and the distribution of false
credentials
o Spoofing of a valid peer and receiving credentials generated for
that peer
o Denial of service
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RFC 5422 Dynamic Provisioning Using EAP-FAST March 20096.1.1. Server-Authenticated Provisioning Mode and Man-in-the-Middle Attacks
In Server-Authenticated Provisioning Mode, the TLS handshake assures
protected communications with the server because the peer must have
been securely pre-provisioned with the trust roots and/or other
authentication information necessary to authenticate the server
during the handshake. This pre-provisioning step prevents an
attacker from inserting themselves as a man-in-the-middle of the
communications. Unfortunately, secure pre-provisioning can be
difficult to achieve in many environments.
Cryptographic binding of inner authentication mechanisms to the TLS
tunnel provides additional protection from man-in-the-middle attacks
resulting from the tunneling of authentication mechanisms.
Server-Authenticated Provisioning Mode provides a high degree of
protection from man-in-the-middle attacks.
6.1.2. Server-Unauthenticated Provisioning Mode and Man-in-the-Middle Attacks
In Server-Unauthenticated Provisioning Mode, the TLS handshake does
not assure protected communications with the server because either an
anonymous handshake is negotiated or the peer lacks the necessary
information to complete the authentication of the server. This
allows an attacker to insert itself in the middle of the TLS
communications.
EAP-FAST Server-Unauthenticated Provisioning Mode mitigates the man-
in-the-middle attack through the following techniques:
o Binding the phase 2 authentication method to secret values derived
from the phase 1 TLS exchange:
In the case of EAP-FAST-MSCHAPv2 used with an anonymous Diffie-
Hellman ciphersuite, the challenges for the EAP-FAST-MSCHAPv2
exchange are derived from the TLS handshake and are not
transmitted within the EAP-FAST-MSCHAPv2 exchange. Since the man-
in-the-middle attack does not know these challenges, it cannot
successfully impersonate the server without cracking the EAP-FAST-
MSCHAPv2 message from the peer before the peer times out.
o Cryptographic binding of secret values derived from the phase 2
authentication exchange with secret values derived from the phase
1 TLS exchange:
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RFC 5422 Dynamic Provisioning Using EAP-FAST March 2009
This makes use of the cryptographic binding exchange defined
within EAP-FAST to discover the presence of a man-in-the-middle
attack by binding secret information obtained from the phase 2
EAP-FAST-MSCHAPv2 exchange with secret information from the phase
1 TLS exchange.
While it would be sufficient to only support the cryptographic
binding to mitigate the MITM, the binding of the EAP-FAST-MSCHAPv2
random challenge derivations to the TLS key agreement protocol
enables early detection of a man-in-the-middle attack. This guards
against adversaries who may otherwise relay the inner EAP
authentication messages between the true peer and server, and it
enforces that the adversary successfully respond with a valid
challenge response.
The ciphersuite used to establish phase 1 of the Server-
Unauthenticated Provisioning Mode MUST be one in which both the peer
and server provide contribution to the derived TLS master key.
Ciphersuites that use RSA key transport do not meet this requirement.
The authenticated and anonymous ephemeral Diffie-Hellman ciphersuites
provide this type of key agreement.
This document specifies EAP-FAST-MSCHAPv2 as the inner authentication
exchange; however, it is possible that other inner authentication
mechanisms to authenticate the tunnel may be developed in the future.
Since the strength of the man-in-the-middle protection is directly
dependent on the strength of the inner method, it is RECOMMENDED that
any inner method used provide at least as much resistance to attack
as EAP-FAST-MSCHAPv2. Cleartext passwords MUST NOT be used in
Server-Unauthenticated Provisioning Mode. Note that an active man-
in-the-middle attack may observe phase 2 authentication method
exchange until the point that the peer determines that authentication
mechanism fails or is aborted. This allows for the disclosure of
sensitive information such as identity or authentication protocol
exchanges to the man-in-the-middle attack.
6.2. Dictionary Attacks
It is often the case that phase 2 authentication mechanisms are based
on password credentials. These exchanges may be vulnerable to both
online and off-line dictionary attacks. The two provisioning modes
provide various degrees of protection from these attacks.
In online dictionary attacks, the attacker attempts to discover the
password by repeated attempts at authentication using a guessed
password. Neither mode prevents this type of attack by itself.
Implementations should provide controls that limit how often an
attacker can execute authentication attempts.
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RFC 5422 Dynamic Provisioning Using EAP-FAST March 2009
In off-line dictionary attacks, the attacker captures information
that can be processed off-line to recover the password. Server-
Authenticated Provisioning Mode provides effecting mitigation because
the peer will not engage in phase 2 authentication without first
authenticating the server during phase 1. Server-Unauthenticated
Provisioning Mode is vulnerable to this type of attack. If, during
phase 2 authentication, a peer receives no response or an invalid
response from the server, then there is a possibility there is a man-
in-the-middle attack in progress. Implementations SHOULD log these
events and, if possible, provide warnings to the user.
Implementations are also encouraged to provide controls, which are
appropriate to their environment, that limit how and where Server-
Unauthenticated Provisioning Mode can be performed. For example, an
implementation may limit this mode to be used only on certain
interfaces or require user intervention before allowing this mode if
provisioning has succeeded in the past.
Another mitigation technique that should not be overlooked is the
choice of good passwords that have sufficient complexity and length
and a password-changing policy that requires regular password
changes.
6.3. Considerations in Selecting a Provisioning Mode
Since Server-Authenticated Provisioning Mode provides much better
protection from attacks than Server-Unauthenticated Provisioning
Mode, Server-Authenticated Provisioning Mode SHOULD be used whenever
possible. The Server-Unauthenticated Provisioning Mode provides a
viable option as there may be deployments that can physically confine
devices during the provisioning or are willing to accept the risk of
an active dictionary attack. Further, it is the only option that
enables zero-touch provisioning and facilitates simpler deployments
requiring little to no peer configuration. The peer MAY choose to
use alternative secure out-of-band mechanisms for PAC provisioning
that afford better security than the Server Unauthenticated
Provisioning Mode.
6.4. Diffie-Hellman Groups
To encourage interoperability implementations of EAP-FAST, anonymous
provisioning modes MUST support the 2048-bit group "14" in [RFC3526].
6.5. Tunnel PAC Usage
The basic usage of the Tunnel PAC is to establish the TLS tunnel. In
this operation, it does not have to provide user authentication as
user authentication is expected to be carried out in phase 2 of EAP-
FAST. The EAP-FAST Tunnel PAC MAY contain information about the
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RFC 5422 Dynamic Provisioning Using EAP-FAST March 2009
identity of a peer to prevent a particular Tunnel PAC from being used
to establish a tunnel that can perform phase 2 authentication other
peers. While it is possible for the server to accept the Tunnel PAC
as authentication for the peer, many current implementations do not
do this. The ability to use PAC to authenticate peers and provide
authorizations will be the subject of a future document. [RFC5077]
gives an example PAC-Opaque format in the Recommended Ticket
Construction section.
6.6. Machine Authentication PAC Usage
In general, the Machine Authorization PAC is expected to provide the
minimum access required by a machine without a user. This will
typically be a subset of the privilege a registered user has. The
server provisioning the PAC should include information necessary to
validate it at a later point in time. This would include expiration
information. The Machine Authentication PAC includes a key so it can
be used as a Tunnel PAC. The PAC-Key MUST be kept secret by the
peer.
6.7. User Authorization PAC Usage
The User Authorization PAC provides the privilege associated with a
user. The server provisioning the PAC should include the information
necessary to validate it at a later point in time. This includes
expiration and other information associated with the PAC. The User
Authorization PAC is a bearer credential such that it does not have a
key that used to authenticate its ownership. For this reason, this
type of PAC MUST NOT be sent in the clear. For additional
protection, the PAC MAY be bound to a Tunnel PAC used to establish
the TLS tunnel. On the peer, the User Authorization PAC SHOULD only
be accessible by the user for which it is provisioned.
6.8. PAC Storage Considerations
The main goal of EAP-FAST is to protect the authentication stream
over the media link. However, host security is still an issue. Some
care should be taken to protect the PAC on both the peer and server.
The peer must securely store both the PAC-Key and PAC-Opaque, while
the server must secure storage of its security association context
used to consume the PAC-Opaque. Additionally, if alternate
provisioning is employed, the transportation mechanism used to
distribute the PAC must also be secured.
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RFC 5422 Dynamic Provisioning Using EAP-FAST March 2009
Most of the attacks described here would require some level of effort
to execute: conceivably greater than their value. The main focus
therefore, should be to ensure that proper protections are used on
both the peer and server. There are a number of potential attacks
that can be considered against secure key storage such as:
o Weak Passphrases
On the peer side, keys are usually protected by a passphrase. In
some environments, this passphrase may be associated with the
user's password. In either case, if an attacker can obtain the
encrypted key for a range of users, he may be able to successfully
attack a weak passphrase. The tools are already in place today to
enable an attacker to easily attack all users in an enterprise
environment through the use of email viruses and other techniques.
o Key Finding Attacks
Key finding attacks are usually mentioned in reference to web
servers where the private Secure Socket Layer (SSL) key may be
stored securely, but at some point, it must be decrypted and
stored in system memory. An attacker with access to system memory
can actually find the key by identifying their mathematical
properties. To date, this attack appears to be purely theoretical
and primarily acts to argue strongly for secure access controls on
the server itself to prevent such unauthorized code from
executing.
o Key duplication, Key substitution, Key modification
Once keys are accessible to an attacker on either the peer or
server, they fall under three forms of attack: key duplication,
key substitution, and key modification. The first option would be
the most common, allowing the attacker to masquerade as the user
in question. The second option could have some use if an attacker
could implement it on the server. Alternatively, an attacker
could use one of the latter two attacks on either the peer or
server to force a PAC re-key, and take advantage of the potential
MITM/dictionary attack vulnerability of the EAP-FAST Server-
Unauthenticated Provisioning Mode.
Another consideration is the use of secure mechanisms afforded by the
particular device. For instance, some laptops enable secure key
storage through a special chip. It would be worthwhile for
implementations to explore the use of such a mechanism.
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